1. Field of the Invention
This invention relates to a method for the manufacture of shaped hollow bodies made of material capable of being poured (cast) and hardened in a hollow mold whose core is provided with an elastic membrane.
Though this method can be used for the manufacture of hollow bodies of any shape, in the following the method is explained with particular reference to the manufacture of air tires made of polyurethane. Thereby, either the casings (carcasses) alone are made of polyurethane, or the casing and the contact area of the tire are simultaneously made of polyurethane in the so-called "Injection-method".
2. Description of the Prior Art
In the manufacture of tires, the pouring of the casing without bubbles is a great problem i.e. the pouring process must be done in such a manner that air-filled inclusions will not occur.
This exclusion of bubbles could, up to now, be achieved only by using relatively complicated costly methods.
It has been suggested to evacuate the die cavity before filling in the cavity with polyurethane. However, this requires that the cavity be very well sealed, so that no air can re-enter the cavity until the pouring process is ended. Such a well-sealed die cavity results in a very high tooling cost. Furthermore, the seal is susceptible to distortions which cannot be avoided due to the repeated heating and cooling of the form. The seal is also subject to shock and vibration which also makes it difficult to maintain tightness.
From the German Published Non-Prosecuted Application No. 2 231 780 for the manufacture of air-tires, it is known to carry out the pouring process while rotating the mold (die form) at the same time. Due to the generated centrifugal forces, a pressure field is created within the poured material so that air bubbles can quickly and reliably escape to the outside and the finished air tire is practically without bubbles. This method also requires high construction costs, because the die and its bearing supports must be very massively constructed due to the relative high centrifugal forces which occur at a rotation number of two revolutions per second.
In principle the possibility exists of filling the die cavity as slowly as possible, and thereby keeping the viscosity of the material which is to be cast and hardened as low as possible. Then the air due to its buoyancy, can slowly escape in the form of bubbles from the cavity. This procedure requires very long reaction times of the material being poured, and an uneconomical long time until the mold can be opened, both of which are unfavorable factors with respect to the manufacturing cost of the tire. Furthermore, for this method only certain polyurethane materials can be used, so that the choice of starting materials is narrow.